An anti-corrosion pretreatment coating is often applied to metal substrates, especially metal substrates that contain iron such as steel, prior to the application of a protective or decorative coating. The pretreatment coating minimizes the amount of corrosion to the metal substrate, if and when, the metal substrate is exposed to moisture and oxygen. Many of the present pretreatment coating compositions are based on metal phosphates, and rely on a chrome-containing rinse. The metal phosphates and chrome rinse solutions produce waste streams that are detrimental to the environment. As a result, there is the ever-increasing cost associated with their disposal. Thus, there is a desire to develop pretreatment coating compositions and methods of applying such compositions without producing metal phosphate and chrome waste solutions. It is also preferred, that these pretreatment coating compositions be effective in minimizing corrosion on a variety of metal substrates because many objects of commercial interest contain more than one type of metal substrate. For example, the automobile industry often relies on metal components that contain more than one type of metal substrate. The use of a pretreatment coating composition effective for more than one metal substrate would provide a more streamlined manufacturing process.
The coating compositions of the present invention are called pretreatment coatings because they are typically applied after the substrate has been cleaned and before the various decorative coatings have been applied. In the automotive industry these decorative coatings often comprise the following layers in order from the substrate out: a pretreatment coating for corrosion resistance, an electrodeposited electrocoat, then a primer layer, a base coat paint, and then a top clear coat. One such pretreatment coating is the Bonderite® system available from Henkel Adhesive Technologies. The Bonderite® systems are conversion coatings that are zinc-phosphate based and include zinc, nickel, manganese and phosphate. Currently, Bonderite® 958 is a standard conversion coating used extensively in the automotive industry. In attempts to move away from conversion coatings that include heavy metals and that produce phosphate waste streams a new class of environmentally friendly conversion coatings have been created. These are exemplified by the TecTalis® line of coatings available from Henkel Adhesive Technologies, certain Oxsilan® products available from Chemetall GmbH and the Zircobond® line from PPG Industries, which are based on a zirconium coating technology, have no phosphates and no nickel or manganese. In particular TecTalis® 1800 is finding increasing use in the automotive industry as a pretreatment coating. While the new zirconium based coatings provide adequate protection for most applications, paint adhesion and corrosion resistance for some applications is not as effective as with the old zinc-phosphate based coatings and solutions to this problem have not been forthcoming.
It is desirable to provide increasing functionality in terms of enhanced corrosion protection, improved paint adhesion, and thinner layers in pretreatment coatings. It is desirable to develop this enhanced functionality in a zirconium based pretreatment coating composition for the reasons noted above related to their reduced environmental issues. At the same time these improvements preferably to do not require changes to existing industrial processing lines and procedures thereby allowing the new pretreatment coating composition to be readily substituted into existing processes.